How do I correct this error: AttributeError: 'NoneType' object has no attribute 'id'

[Maurice Waka]

I have a small code with this:

db.define_table('book',                         

                Field('name',requires = IS_NOT_EMPTY()),

                Field('category',

                      requires = IS_IN_SET(['Politics','Finance','Fiction','Wellness']),

                      default = 'Politics'),        

                Field('price', 'integer',requires = IS_NOT_EMPTY()),

                Field('image','upload'),

                Field('selling_type',requires = IS_IN_SET(['Bids','Buy']),default = 'Bid'),

                Field('book_author',

                      db.auth_user,

                      default=auth.user_id,

                      writable=False,

                      readable=False,

                      requires=IS_IN_DB(db, db.auth_user.id,'%(first_name)s')),

                Field('description','text',requires = IS_NOT_EMPTY()),

                Field('created_on','datetime',

                      default=request.now,

                      update=request.now,

                      writable=False))

product = db(book_author.id==db.book.book_author).select(db.book.ALL)

But I get this Error:

    product = db(book_author.id==db.book.book_author).select(db.book.ALL)

AttributeError: 'NoneType' object has no attribute 'id'

[Maurice Waka]

Seems to work after restarting the system